1. Field of the Invention
The present invention relates to an apparatus for detecting trouble in an exhaust-gas recirculation (hereinafter referred to as "EGR") system of an internal combustion engine, particularly to an apparatus capable of detecting clogging occurred in the EGR passage provided with an EGR control valve. More specifically, it relates to an apparatus for detecting trouble in the EGR system, capable of discriminating whether clogging has occurred on the upstream side or on the downstream side of the EGR control valve.
2. Description of the Related Art
To reduce an amount of NOx in an exhaust gas, an EGR system, as is well known, wherein the exhaust gas flowing through an exhaust passage is recirculated to an intake passage of an internal combustion engine via an EGR passage, is used. In the EGR system of such a type, an EGR control valve is usually provided in the EGR passage. The amount of EGR gas to be supplied to the intake passage of the internal combustion engine is controlled by the opening degree of the EGR control valve. However, the recirculation of exhaust gas may be continuously interrupted if the EGR control valve is broken or the EGR passage clogs. If such an inconvenience is left as it is a large amount of NOx is continuously discharged because no EGR is conducted. In addition, the reduction of the amount of recirculating exhaust gas or the interruption of EGR due to the trouble in the EGR system is seldom found by the driver.
To solve such a problem, apparatuses for diagnosing trouble in the EGR system have been proposed wherein the opening degree of EGR control valve is controlled by the magnitude of a negative pressure introduced into a diaphragm chamber in the EGR control valve partitioned by a diaphragm. For example, Japanese Unexamined Patent Publication (Kokai) No. 63-75345 discloses an apparatus diagnosing the trouble of EGR system of a diaphragm type comprising a pressure detection means provided upstream of EGR control valve and means for detecting the supply of negative pressure to the EGR control valve. According to the trouble detection apparatus disclosed in the above Patent Publication, "trouble" is determined if no flow of exhaust gas is detected by the pressure detection means when a negative pressure is supplied to the EGR control valve. Contrarily, "trouble" is also determined if a flow of exhaust gas is not detected when the negative pressure is supplied to the EGR control valve.
According to this apparatus, however, although trouble in the EGR system can be detected, there is a problem in that it is impossible to determine whether the trouble is caused by the malfunction of the EGR control valve or by clogging of the EGR passage.
Also, according to the proposed apparatus, pressure-detection means is used for detecting whether or not the exhaust-gas is introduced into the EGR passage. However, since the pressure in the exhaust system varies in accordance with the rotational speeds or loads of the engine, it is not apparent that the pressure variation has occurred due to the lack of exhaust gas or due to the variation of the operative condition or of the environment. Accordingly, there has been a problem in the proposed apparatus in that a mis-determination of trouble in the EGR system may occur.